1. Field of the Invention
This invention relates to refrigeration systems in which a pump is used to transfer liquid or mixed phased refrigerant. More specifically, the invention relates to refrigeration systems, and to the pump therefor, in which the pump comprises a positive displacement, sealless, balanced rotor, vane pump.
2. State of the Prior Art
During the summer season electrically driven air conditioning and refrigeration systems place a heavy load on utility power grids during midday hours. This electric capacity is often provided by less efficient, yet low capital cost, power generating equipment. Accordingly, this "peak power" is more expensive to generate per unit of energy. Thus, many utilities charge a premium for electrical power during these peak periods. During the late evening and night when many industrial electrical users are not operating and when air conditioning loads are at their minimum, electrical demand is usually also at a minimum on the utility's power grid. Thus, some electric utilities actually discount power sales during these "off-peak" periods.
The higher "on-peak" price charged for electricity during the highest periods of air conditioning demand increases the overall cost for providing air conditioning in most installations. In response, many designers of air conditioning equipment have endeavored to provide some form of negative energy storage into their air conditioning system whereby the refrigeration equipment can be operated during "off-peak" hours to chill a mass of storage media which can later be used during the "on-peak" periods of the day as a heat sink to draw heat out of an air conditioned space. Typically, such storage media comprises a liquid experiencing some form phase change, such as freezing, to increase its negative energy storage density.
One such an air conditioning system is described in the Uselton et al. U.S. Pat. No. 5,211,029 issued May 18, 1993 and incorporated herein by reference. The Uselton et al. patent discloses a standard freon based compressor driven condensing and evaporating type refrigeration system into which has been incorporated a tank of negative energy storage media. Coils are provided to circulate the refrigerant of the refrigeration system through the tank of negative energy storage media. A transfer pump is provided for drawing condensed and chilled refrigerant from the tank of negative energy storage media and passing it through the evaporator in the refrigeration system.
Another type of air conditioning or refrigeration system is the liquid overfeed system in which excess liquid refrigerant is forced through evaporators to effect cooling. These systems often use pumps to circulate refrigerant. However, the system must be carefully designed and may require more controls to ensure that the pump has adequate subcooled liquid refrigerant available to effect cooling since centrifugal or turbine pumps are often used.
Pumps used in this system must be able to run continuously over a long period of time, be relatively long-lived without breakdowns, must be efficient in operation, and must be able to move mixed phase (gas/liquid) fluids as well as liquid refrigerant. Often, the pump chamber is filled only with vapor phase refrigerant upon start-up so that the pump must be self priming and have a superior dry running capability. Further, such pumps must also be free from leakage of the refrigerant.
Several pumps have been proposed for use in the Uselton et al. refrigeration system. One such pump is a gear pump as the transfer pump. However, most refrigerants typically have very low viscosity and therefore provide insufficient lubrication to prevent rapid wear of moving parts of pumps and compressors. As the gears in a gear pump wear over the life of the pump, the slip of fluid past the rotating gears greatly reduces their efficiency and capacity at a given pressure, especially with such low viscosity pumping liquids.
Centrifugal pumps are often used to pump liquids and have many advantages in this service. However, as the media in the negative energy storage tank warms up, the refrigerant passing through the tank may not be completely condensed and may enter the transfer pump in a mixed phase state. Centrifugal pumps are inappropriate for pumping mixed phase media.
Many types of refrigerants used in evaporative refrigeration systems are potentially harmful to the environment, and newer refrigerants may pose health risks. Also, leakage results in system ineffectiveness. Therefore, it is desirable to minimize all leaks and discharges of refrigerants from the system. Due to its low viscosity, refrigerant is particularly susceptible to leakage past dynamic seals on a pump shaft as it passes through the pump housing.
Due to sliding friction between moving parts, such as rotors, gears, pistons, bearings, etc., pumps and compressors have previously been designed with an oil sump and some means of separation and/or oil return to ensure proper fluid film between parts in relative motion. Typically, a small amount of oil is mixed with the refrigerant to help lubricate moving parts. For some refrigerants, the oil may not be miscible which creates special design problems due to oil fouling of evaporator or condenser tubes, filters, etc. HCFC-22 is particularly miscible with oil, HFC-134a is hardly miscible and ammonia is immiscible with oil.